1. Field of the Invention
This invention relates to an active noise reducing receiver device for converting electrical signals into output acoustic signals. More particularly, it relates to a noise reducing receiver device in which acoustic signals in the vicinity of an electro-acoustic transducer element are converted by a microphone into electrical signals and negatively fed back for noise reduction to an amplifier circuit which is adapted for amplifying input electrical signals and supplying the amplified signals to the electro-acoustic transducer element.
2. Prior Art
Headphone devices in the form of a receiver utilizing only an electro-acoustic transducer element attached to the listener's ear used extensively.
However, with such passive type headphone devices, not only the acoustic output, but also surrounding noise is input to the listener's ear. For this reason, a so-called active type noise-reducing headphone device has recently been proposed, as disclosed for example in U.S. Pat. Nos. 4,455,675 and 4,494,074, according to which the noise in the vicinity of the headphone unit is reduced by a negative feedback loop, by means of which output acoustic signals in the vicinity of the headphone unit which is adapted to output acoustic signals, converted from electrical signals are converted into electrical signals and fed back in an antiphase relation to the input electrical signals.
Referring to FIG. 1, in which the basic construction of the above mentioned active type noise reducing headphone device is illustrated, a microphone unit 6 is provided in the vicinity of a headphone unit 4 attached to a listener's ear 20, and a signal synthesizer 2 is provided at an input side of an amplifier 3 which is adapted for amplifying an input electrical signal S applied from a signal source 10 to a signal input terminal 1 before supplying the signal to the headphone unit 4. The acoustic signals in the vicinity of the headphone unit 4 are converted by the microphone unit 6 into electrical signals which are supplied via feedback circuit 7 to the signal synthesizer 2 where the input electrical signal S and feedback signals output from the feedback circuit 7 are summed together before being supplied to the amplifier 3 as negative feedback.
As in the above described active type noise reducing headphone device, the noise level in the acoustic signal input to the listener's ear 20 may be reduced by summing acoustic output signals from the headphone unit 4 and noise signals from the vicinity of the the acoustic meatus of the listener's ear 20 to produce an acoustic signal, converting the acoustic signal by means of the microphone unit 6 into an electrical signal and negatively feeding back the electrical signal via feedback circuit 7 to the input side of the amplifier 3.
In the conventional passive type headphone device, its frequency characteristics are monistically determined by the size or the weight of the diaphragm of the headphone unit, the impedance characteristics of the voice coil or the acoustic circuit around the diaphragm of the headphone unit. The frequency characteristics can only be corrected by gradually changing various factors influencing the frequency characteristics for achieving the desired characteristics. Moreover, distortion caused by the magnetic circuits or due to mechanical nonlinearities, such as edges, occur frequently. Above all, distortion predominates at the low range resonant frequency not exceeding f.sub.0.
It is noted that, with the use of the active type noise reducing headphone device, whose function is to reduce external noise, too large a noise reduction level may result in a listener hearing a music broadcast while failing to hear another person talking to him or failing to hear an emergency signal. On the other hand, two low a noise reduction level proves to be ineffective in factories or at construction sites with a high noise level.
The conventional active type noise reducing headphone device suffers from the drawback that, since it has fixed characteristics, it cannot be used for certain occasionally desirable noise reduction levels.
The above mentioned conventional active type noise reducing headphone device also has the drawback that, when mechanical vibrations, such as impact vibrations applied to the housing of the headphone device or frictional vibrations of connection cords, are transmitted to the microphone unit, these vibrational noises are converted by the microphone unit into electrical signals, so that external noise cannot be reduced in a regular manner. In other words, the microphone unit, which is adapted for converting the sound pressure into electrical signals, is also responsive to mechanical vibrations to convert mechanical vibration noise into output electrical signals.